This invention relates to a variable displacement axial piston pump and, more particularly, to a means for preventing cavitation in the pump over a wide range of pump speeds.
An axial piston pump has a barrel rotatably mounted within a pump housing. A plurality of equally spaced cylinders are formed at a common radius in the barrel. Each cylinder houses a piston which reciprocates as the barrel is rotated. One end of the barrel lies against a fixed port plate mounted in the housing which contains a pair of sausage-shaped ports. One of the ports is an inlet port and the other is an exhaust port. Each cylinder has a port in the end of the barrel adjacent the port plate. As the barrel is rotated, each cylinder port traverses the inlet port and the exhaust port. As the cylinder ports traverse the inlet port low pressure fluid is drawn into the cylinder. When the cylinder ports traverse the exhaust port, they expel the fluid at an increased pressure.
As the rotational speed of the pump increases, the time during which the cylinder ports traverse the inlet port decreases. If the cylinders are not completely filled with fluid after they traverse the inlet port, cavitation occurs. Noise, vibration and rapid errosion of metal surfaces contacted by the fluid, all of which are objectionable, are caused as a result of cavitation. For these reasons, cavitation must be avoided.
In order for a cylinder to fill as the cylinder port traverses the inlet port, the fluid coming through the inlet port must have a velocity component along the axis of the cylinder. This velocity component is generally derived from the pressure of the fluid coming into the inlet port. Additionally, since the cylinder port moves tangentially relative to the inlet port, it is desirable to have the fluid enter the inlet port with a tangential velocity component as well as an axial component. If the fluid entering the inlet port has a tangential velocity component substantially equal to the tangential velocity of the cylinder port, the time for filling the cylinder is greatly reduced and cavitation is prevented at increased pump speeds. In fact, increasing the tangential velocity component of the fluid has a greater effect on decreasing the time it takes to fill a cylinder than a correspondng increase in the axial velocity component.
One method of reducing cavitation is to precharge or supercharge the fluid coming into the inlet port. This is accomplished by having an auxiliary pump increase the pressure and hence the energy of the fluid above the minimum required to ensure complete filling of the barrel cylinders at all pump operating speeds. Supercharging the incoming fluid by an auxiliary pump has a number of disadvantages. An auxiliary pump adds to the cost of a hydraulic system and also occupies space which is sometimes at a premium. Furthermore, auxiliary pumps are commonly operated to increase the present of the incoming fluid to a level sufficient to fill the barrel cylinders at the maximum operating speed of the pump. However, since a pump is not always operated at its maximum speed, the auxiliary pump is providing supercharged fluid at a greater pressure than is necessary for a portion of the time the pump is operaing, which results in wasted energy.
It is desirable to provide a means for increasing the tangential velocity of the fluid entering the inlet port to substantially equal the tangential velocity of the cylinder port.
It is further desirable to have the tangential velocity of fluid entering the inlet port substantially equal to the tangential velocity of the cylinder ports at each operating speed of the pump. In other words, the means for increasing the tangential velocity of the incoming fluid should be directly proportional to the speed of the pump. It is also desirable to increase the tangential velocity of the incoming fluid without using an auxiliary pump.